Just to let you know, I tried this way back when I loved firefox. I was so firefox crazy I had looked thrugh each and every addon they had, i had about 60 addons.
It was pretty beautiful im not gona lie.
But I tried this and man, It screwed my firefox so bad. maybe cause I had all my addons, but right after i did this, I went right back in and actually increased how much RAM it could take up and it always worked faster. For internal stuff atleast.
So sure if u have 10 or lesss addons, and small amout of unused ram then this would probably be good. But I'd say if you have alot of ram, (2gbs+) then just let firefox have it, if not more.